Two ras-related genes have been isolated from yeast Saccharomyces cerevisiae using mammalian Harvey ras DNA as a probe. These have been named c-ras-sc-1 and c-ras-sc-2. We have obtained a complete DNA sequence of both the yeast ras genes. The DNA sequence predicts encoded polypeptides of 40,000 daltons for c-ras-sc-1 and 41,100 for c-ras-sc-2. Of their N-terminal 90 amino acids; the yeast ras genes retain more than 80% of homology with other mammalian ras genes. Their c-termini, however, have diverged. We have used c-Ha-ras and c-Ki-ras probes to study the expression of these genes at the transcriptional level in a developmental rat mammary gland system. We have observed an increase in the concentration of c-Ha-ras RNA in the mammary gland during gestation, whereas during lactation it drops to about one-fifth of that level. The levels of c-Ki-ras RNA remains constant during the development stage.